Android提供了一个MediaExtractor类，可以用来分离容器中的视频track和音频track，下面的例子展示了使用MediaExtractor和MediaMuxer来实现视频的换音：

private void muxingAudioAndVideo() throws IOException {
    MediaMuxer mMediaMuxer = new MediaMuxer(mOutputVideoPath, 
                MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);

    // 视频的MediaExtractor
    MediaExtractor mVideoExtractor = new MediaExtractor();
    mVideoExtractor.setDataSource(mVideoPath);
    int videoTrackIndex = -1;
    for (int i = 0; i < mVideoExtractor.getTrackCount(); i++) {
        MediaFormat format = mVideoExtractor.getTrackFormat(i);
        if (format.getString(MediaFormat.KEY_MIME).startsWith("video/")) {
            mVideoExtractor.selectTrack(i);
            videoTrackIndex = mMediaMuxer.addTrack(format);
            break;
        }
    }

    // 音频的MediaExtractor
    MediaExtractor mAudioExtractor = new MediaExtractor();
    mAudioExtractor.setDataSource(mAudioPath);
    int audioTrackIndex = -1;
    for (int i = 0; i < mAudioExtractor.getTrackCount(); i++) {
        MediaFormat format = mAudioExtractor.getTrackFormat(i);
        if (format.getString(MediaFormat.KEY_MIME).startsWith("audio/")) {
            mAudioExtractor.selectTrack(i);
            audioTrackIndex = mMediaMuxer.addTrack(format);
        }
    }

    // 添加完所有轨道后start
    mMediaMuxer.start();

    // 封装视频track
    if (-1 != videoTrackIndex) {
        MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();
        info.presentationTimeUs = 0;
        ByteBuffer buffer = ByteBuffer.allocate(100 * 1024);
        while (true) {
            int sampleSize = mVideoExtractor.readSampleData(buffer, 0);
            if (sampleSize < 0) {
                break;
            }

            info.offset = 0;
            info.size = sampleSize;
            info.flags = MediaCodec.BUFFER_FLAG_SYNC_FRAME;
            info.presentationTimeUs = mVideoExtractor.getSampleTime();
            mMediaMuxer.writeSampleData(videoTrackIndex, buffer, info);

            mVideoExtractor.advance();
        }
    }

    // 封装音频track
    if (-1 != audioTrackIndex) {
        MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();
        info.presentationTimeUs = 0;
        ByteBuffer buffer = ByteBuffer.allocate(100 * 1024);
        while (true) {
            int sampleSize = mAudioExtractor.readSampleData(buffer, 0);
            if (sampleSize < 0) {
                break;
            }

            info.offset = 0;
            info.size = sampleSize;
            info.flags = MediaCodec.BUFFER_FLAG_SYNC_FRAME;
            info.presentationTimeUs = mAudioExtractor.getSampleTime();
            mMediaMuxer.writeSampleData(audioTrackIndex, buffer, info);

            mAudioExtractor.advance();
        }
    }

    // 释放MediaExtractor
    mVideoExtractor.release();
    mAudioExtractor.release();

    // 释放MediaMuxer
    mMediaMuxer.stop();
    mMediaMuxer.release();
}

MediaExtractor的接口比较简单，首先通过setDataSource()设置数据源，数据源可以是本地文件地址，也可以是网络地址：

MediaExtractor mVideoExtractor = new MediaExtractor();
mVideoExtractor.setDataSource(mVideoPath);

然后可以通过getTrackFormat(int index)来获取各个track的MediaFormat，通过MediaFormat来获取track的详细信息，如：MimeType、分辨率、采样频率、帧率等等：

for (int i = 0; i < mVideoExtractor.getTrackCount(); i++) {
    MediaFormat format = mVideoExtractor.getTrackFormat(i);
}

获取到track的详细信息后，通过selectTrack(int index)选择指定的通道：

if (format.getString(MediaFormat.KEY_MIME).startsWith("video/")) {
    mVideoExtractor.selectTrack(i);
    break;
}

指定通道之后就可以从MediaExtractor中读取数据了：

while (true) {
    int sampleSize = mVideoExtractor.readSampleData(buffer, 0);
    if (sampleSize < 0) {
        break;
    }
    // do something

    mVideoExtractor.advance();  // 移动到下一帧
}

在读取结束之后，记得释放资源：

mVideoExtractor.release();